Anticholinergic Effects

The tricyclics block muscarinic receptors and can cause a variety of anticholinergic side effects, such as dry mouth, constipation, blurred vision, and urinary hesitancy. These effects can precipitate an ocular crisis in patients with narrow-angle glaucoma. The tricyclic and tetracyclic compounds vary substantially in their muscarinic potency (see Table 12-1). Amitriptyline is the most potent, followed by clomipramine. Of the tricyclics, desipramine is the least anticholinergic. Amoxapine and maprotiline also have minimal anticholinergic effects. Anticholinergic effects can contribute to tachycardia, but tachycardia also occurs as a result of stimulation of ^-adrenergic receptors in the heart. Thus, tachycardia regularly occurs in patients receiving desipramine, which is minimally anticholinergic (Rosenstein and Nelson 1991).

Although anticholinergic effects may be annoying, they are usually not serious. They can, however, become severe. An ocular crisis in patients with narrow-angle glaucoma is an acute condition associated with severe pain. Urinary retention can be associated with stretch injuries to the bladder. Constipation can progress to severe obstipation. (Paralytic ileus has been described but is rare.) In these conditions, medication must be discontinued and appropriate supportive measures instituted. Elderly patients are at greatest risk for severe adverse consequences. The frequency of severe anticholinergic adverse reactions is increased by concomitant neuroleptic administration. Use of nortriptyline or desipramine, either of which is less anticholinergic, can help to reduce the likelihood of these problems.

Anticholinergic effects may benefit from other interventions. Bethanechol (Urecholine) at a dosage of 25 mg three or four times a day may be helpful in patients with urinary hesitancy. The regular use of stool softeners helps to manage constipation. Patients with narrow-angle glaucoma who are receiving pilocarpine eyedrops regularly can be treated with a tricyclic, as can those who have had an iridectomy. Tricyclic agents do not affect patients with chronic open-angle glaucoma.

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